Gas turbine engines for commercial aircraft applications typically include an engine core housed within a core nacelle. In one type of arrangement known as a turbofan engine, the core drives a large fan upstream from the core that provides airflow into the core. A significant portion of airflow bypasses the core to provide thrust. One or more spools are arranged within the core, and a gear train may be provided between one of the spools and the fan. A fan case and fan nacelle surround the fan and at least a portion of the core.
A gas turbine propulsion systems includes a core engine that drives a fan. A fan nacelle surrounds the fan. The nacelle has a rounded forward construction often referred to as the inlet cowl. One surface of this cowl forms an inner surface of the cowl and additionally forms the outer perimeter of the engine flow stream into the downstream fan. A second surface of this cowl creates an outer surface forming the exterior of the nacelle. The two surfaces of the cowl are contiguous and connected through a rounded leading edge segment often referred to as an inlet lip. During engine operation, it is desired to have limited flow separation about this cowl and to the flow entering the engine. Flow separation occurs where airflow communicated through the inlet lip separates from an inner surface of the inlet lip portion. Flow separation can introduce flow distortion at the inlet that affects engine performance including efficiency and stall margin. Flow distortion at the exterior of the nacelle can promote drag and other undesirable impacts to the aircraft system as a whole.
Accordingly, it is desirable to improve the performance of a turbofan gas turbine engine by limiting flow separation at the fan nacelle.